Muscarinic receptor regulation in the sympathetically innervated avian expansor secundariorum

We sought to examine further the regulation of muscarinic receptors in the developing expansor secundariorum, a smooth muscle of the avian wing. [(3)H]Quinuclidinyl benzilate binding was used as a measure of muscarinic receptors present in the muscle, which gradually decline in density from hatch during the following few weeks. Receptor loss can be prevented by denervation of the muscle immediately after hatch, but receptor density is not recovered by denervation in older birds. As the major innervation is provided by sympathetic, noradrenergic fibres with no evidence of a cholinergic input, the action of several pharmacological agents known to affect noradrenergic transmission was examined. Reserpine partially prevented the early receptor decline but phenoxybenzamine and dimethylphenylpiperazinium did not significantly affect receptor density. The muscle was also immobilized by tenotomy, since this unique muscle is attached to the shoulder by a long, discrete tendon. Tenotomy significantly retarded the normal developmental receptor loss. It is suggested that in this muscle, muscarinic receptor density is regulated by noradrenergic nerves, possibly by a mechanism involving the transmitter, noradrenaline.